[fitsbits] FITS 'keyword dictionaries'

[Joe Hourcle]

On Wed, 2 Apr 2014, Tom Kuiper wrote:

> On 04/02/2014 02:10 PM, Erik Bray wrote:
>> I wasn't intending to broadcast this to the broader community yet, so I won't go
>> into too much detail here.  I just decided to go ahead and bring it up since it
>> was relevant to your search for related efforts.  The documentation for the
>> current prototype can be read here:
>>
>> http://embray.github.io/PyFITS/schema/users_guide/users_schema.html
>>
>> A prototypical example of a schema can be seen here:
>>
>> https://github.com/embray/PyFITS/blob/standard-keywords/lib/pyfits/hdu/base.py#L71
> Hi Erik and Joe,
>
> I'd just like to register interest in this discussion, in case it
> continues off-line.
>
> My interest is in using FITS headers to record faithfully the equipment
> configuration used during an observing session.  True, FITS was designed
> as a data transport format, but a conscientious analyst wants to know
> pretty much everything about how the data were gathered.  For the
> purpose that I have in mind, current usage is pretty inadequate.
> Consider, for example, the paucity of information conveyed by TELESCOP
> and INSTRUME.

I've actually had some other thoughts regarding those two keywords 
specifically.

I was thinking that much like the 'file' command in UNIX, it might be 
worth making a list of values in use, so that if someone had some 
random FITS file, they could run it through a program that would attempt 
to identify the file, and possibly give a reference of where to find more 
information about that particular instrument.

(and even better, check with the authoritative archive to see if the 
file's still current, or if it's been deprecated by a better calibration 
... but then I start getting really far ahead of myself)


> In moving ahead I will invent new keywords, which most FITS readers will
> hopefully ignore but which a custom Python program could extract.  I
> want to be sure that I stay close to the generally accepted meanings of
> the existing keywords.   In "generally accepted" lies the problem, so
> I'll be interested in what you come up with.

I'm not sure that there really is a 'generally accepted' for some of these 
keywords.

For solar physics, INSTRUME is the investigation name, and TELESCOP could 
be the same, the name of the mission, or the name of the specific detector 
used by the investigation.

So, for SOHO/LASCO (a package of three coronagraphs) we have:

 	TELESCOP= 'SOHO    '           /
 	INSTRUME= 'LASCO   '           /
 	DETECTOR= 'C2      '           /

For STEREO/SECCHI (3 telescopes + 2 coronagraphs on two different 
spacecraft), we have:

 	DETECTOR= 'HI2     '           /
 	INSTRUME= 'SECCHI  '           /
 	OBSRVTRY= 'STEREO_A'           /
 	TELESCOP= 'STEREO  '           /

... and we have cases where INSTRUME is changed depending on the optical 
path through the instrument ... so Hinode/SOT as a filtergram:

 	TELESCOP= 'HINODE'
 	INSTRUME= 'SOT/WB'

... vs. Hinode/SOT as a 4D spectropolarimeter:

 	TELESCOP= 'HINODE'
 	INSTRUME= 'SOT/SP'

... but those are all from spacecraft .. there's much more variability 
from ground-based observatories:

 	INSTRUME= 'CLIMSO C1'          / Name of the instrument
 	CAMERA  = 'U4000   '           / Name of the CCD camera

 	ORIGIN  = 'MT. WILSON'          /
 	TELESCOP= '60 FT'               /

 	INSTRUME= 'NRH ROUT'         /

 	TELESCOP= 'NRH'                / Nancay Radioheliograph
 	INSTRUME= 'NRH2'               / Nancay 2D-images Radioheliograph

 	ORIGIN  = 'National Solar Observatory -- GONG' / FITS file originator
 	OBS-SITE= 'NSO/GONG NETWORK'   / Instrument Site location
 	TELESCOP= 'NSO-GONG'           / NSO/GONG Network

 	ORIGIN  = 'BBSO'               / BIG BEAR LAKE, CA
 	TELESCOP= '26W'                /
 	CAMERA  = '1.4i'               / KODAK MODEL NUMBER; SHUTTER DIRECTION:  1
 	OBSERVER= 'BBSO'               /


But in some cases, neither of those is present, but we have other 
keywords that might be useful for identification:

 	COMMENT    NSO/SP/ESF spectroheliogram

 	MPROGNAM= 'TR_REFORMAT.PRO'    /
 	PROG_NAM= 'TR_WRT_FITS_I1'     /Make a FITS file per hour

 	LAMBDA  =              6562.80 /(float) Wavelength of Image [A]
 	ARCS_PP =            0.0710000 /(float) Image Scale [arcs/pixel]
 	TEL_DIA =              43.8000 /(integer) Telescope aperture [cm]

 	CREATOR = 'BASS2000 Full Sun - Meudon' /


... this has been a problem for the Virtual Solar Observatory, as try to 
work with the different archives to determine how we should register 
different collections in our system so that a search on an 'instrument' 
will give an expected result.

(we cheat some ... if the match for 'instrument' comes up empty, we swap 
'instrument' and 'detector' and run the search again ... so searching for 
'instrument=c1' will match both CLIMSO/C1 and LASCO/C1)

As I don't think it's realistic for archives to go back and change their 
headers, part of the reason for this documentation is so we can fill in 
constants and synonyms... or maybe even specify how missing keywords 
should be derived from existing ones.

-Joe

ps.  I'm not sure if I should also bring this up on VOTable mailing lists 
... I would assume that a solution that could document both formats would 
be preferable.

-----
Joe Hourcle
Programmer/Analyst
Solar Data Analysis Center
Goddard Space Flight Center